The present invention relates to liquid-crystalline compounds and to a liquid-crystalline medium, to its use for electro-optical purposes and to displays containing said medium.
Liquid crystals are used especially as dielectrics in display devices, as the optical properties of such substances can be affected by an applied voltage. Electro-optical devices on the basis of liquid crystals are very well known to those skilled in the art and can be based on various effects. Examples of such devices include cells with dynamic scattering, DAP cells (deformation of aligned phases), guest/host cells, TN cells having a twisted nematic structure, STN cells (supertwisted nematic), SBE cells (superbirefringence effect) and OMI cells (optical mode interference). The most common display devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid crystal materials must have good chemical and thermal stability and good stability with respect to electrical fields and electromagnetic radiation. Additionally, the liquid crystal materials should have a low viscosity and give rise to short response times, low threshold voltages and high contrast in cells.
Furthermore they should, at standard operating temperatures, i.e., in as wide a range as possible below and above room temperature, have a suitable mesophase, for example a nematic or cholesteric mesophase for the abovementioned cells. Since liquid crystals as a rule are used as mixtures of a number of components, it is important for the components to be readily miscible with one another. Other properties such as electrical conductivity, dielectric anisotropy and optical anisotropy must meet various requirements, depending on the cell type and field of application. For example, materials for cells having a twisted nematic structure should exhibit positive dielectric anisotropy and low electrical conductivity.
Matrix liquid crystal displays, for example, comprising integrated nonlinear elements to switch individual pixels (matrix LCDs) ideally require media having large positive dielectric anisotropy, broad-range nematic phases, relatively low birefringence, very high resistivity, good UV and temperature stability and low vapor pressure.
Such matrix liquid crystal displays are known. Suitable nonlinear elements for individually switching the separate pixels include active elements (i.e. transistors), for example. Such an arrangement is referred to as an xe2x80x9cactive matrixxe2x80x9d, allowing for a distinction between two types:
1. MOS (metal oxide semiconductor) or other diodes on a silicon wafer as the substrate.
2. Thin-film transistors (TFT) on a glass sheet as the substrate.
The use of monocrystalline silicon as a substrate material limits the display size, since even modular assembly of separate subdisplays gives rise to problems at the joints.
In the more promising type 2, which is preferred, the electro-optical effect used is usually the TN effect. A distinction is made between two technologies: TFTs composed of compound semiconductors such as CdSe, or TFTs on the basis of polycrystalline or amorphous silicon. Work on the latter technology is being carried out worldwide with great intensity.
The TFT matrix is applied on the inside of the one glass sheet of the display, while the other glass sheet on its inside carries the transparent counter-electrode. Compared with the size of the pixel electrode, the TFT is very small and hardly interferes with the image. This technology can also be extended to full color capability pictorial representations, where a mosaic of red, green and blue filters is arranged in such a way that filter elements are located opposite switchable picture elements in a one-to-one arrangement.
The TFT displays usually function as TN cells comprising crossed polarizers in transmission and employ backlighting.
The term matrix LCDs in this context encompasses any matrix display comprising integrated nonlinear elements, i.e. in addition to the active matrix it also includes displays comprising passive elements such as varistors or diodes (MIM=metal-insulator-metal).
Matrix LCDs of this type are suitable, in particular, for TV applications (e.g. portable televisions) or for high information level displays for computer applications (laptop) and in motor vehicle or aircraft production. In addition to problems regarding angular dependence of contrast and switching times, matrix LCDs present difficulties owing to insufficiently high resistivity of the liquid crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 et seq., Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145 et seq., Paris]. As the resistance decreases, the contrast of a matrix LCD display deteriorates, and the problem of xe2x80x9cafterimage eliminationxe2x80x9d can arise. As the resistivity of the liquid crystal mixture generally decreases over the lifetime of a matrix LCD, owing to interaction with the interior surfaces of the display, a high (initial) resistance is very important to achieve acceptable service life. Particularly with low-voltage mixtures it has hitherto been impossible to achieve very high resistivities. Moreover, it is important for the resistivity to exhibit as low an increase as possible with increasing temperature and after thermal exposure and/or exposure to UV. A further particularly disadvantageous feature is the low-temperature properties of the prior art mixtures. It is desirable that no crystallization and/or smectic phases occur even at low temperatures and that viscosity temperature dependence be as small as possible. The prior art matrix LCDs therefore do not meet present-day requirements.
Therefore a great need is still present for matrix LCDs having very high resistivity and at the same time having a wide operating temperature range, short switching times even at low temperatures, and a low threshold voltage, which do not exhibit the drawbacks of the prior art or exhibit them only to a lesser extent.
For TN (Schadt-Helfrich) cells, media are desirable which permit the following advantages in these cells:
extended nematic phase domain (especially towards low temperatures)
switchability at extremely low temperatures (outdoor use, motor vehicles, avionics),
increased resistance to UV radiation (extended lifetime), and
low optical birefringence.
The media available from the prior art do not permit these advantages to be achieved while at the same time maintaining other parameters.
For supertwisted cells (STN), media are desirable which permit higher multiplexability and/or lower threshold voltages and/or wider nematic phase domains (especially at low temperatures). For this purpose, a further expansion of the available parameter space (clearing point, transition smectic-nematic or melting point, viscosity, dielectric parameters, elastic parameters) is urgently required.
It is an object of the invention to provide media especially for such matrix LCDs, TN or STN displays which do not exhibit the above-mentioned drawbacks or exhibit them only to a lesser extent, and preferably at the same time have very high resistivities and low threshold voltages. This object requires liquid-crystalline compounds having a high clearing point and low rotational viscosity.
We have found that this object can be achieved if the liquid-crystalline compounds according to the invention are employed.
The invention therefore relates to liquid-crystalline compounds of formula I, 
wherein
R1 is a straight-chain or branched alkyl radical having 1 to 15 C atoms which is unsubstituted, singly substituted by CN or CF3, at least singly substituted by halogen, wherein optionally one or more CH2 groups are substituted by xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94OCxe2x80x94Oxe2x80x94 or xe2x80x94Oxe2x80x94COxe2x80x94 in such a way that O atoms are not directly linked together,
R2 is CN, SF5, H, F, Cl, NCS, SCN, or a straight-chain or branched alkyl radical having 1 to 15 C atoms which is unsubstituted, singly substituted by CN or CF3, at least singly substituted by halogen, wherein optionally one or more CH2 groups are substituted by xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94OCxe2x80x94Oxe2x80x94 or xe2x80x94Oxe2x80x94COxe2x80x94 in such a way that O atoms are not directly linked together,
A1, A2, A3 and A4 are each, independently, a 1,4-cyclohexenylene radical in which one or two non-adjacent CH2 groups are optionally replaced by xe2x80x94Oxe2x80x94 or xe2x80x94Sxe2x80x94, a 1,4-phenylene radical in which one or two CH groups are optionally replaced by N, or a radical selected from piperidine-1,4-diyl, 1,4-bicyclo[2.2.2]octylene, naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl, and 1,2,3,4-tetrahydronaphthalene-2,6-diyl, wherein each is optionally singly substituted or polysubstituted by halogen,
Z1 and Z2 are each, independently, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94(CH2)4xe2x80x94, xe2x80x94C2F4xe2x80x94, xe2x80x94CF2CH2xe2x80x94, xe2x80x94CH2CF2xe2x80x94, xe2x80x94CFxe2x95x90CFxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94 or a single bond,
a is 0, 1 or 2,
b is 0, 1 or 2, and
c is 0, 1 or 2,
wherein a+b+cxe2x89xa62.
The invention further relates to the use of the compounds of formula I in liquid-crystalline media.
The compounds of formula I have a wide application range. Depending on the choice of substituents, these compounds can serve as base materials for liquid-crystalline media. Alternatively, compounds of the formula I can also be admixed to liquid-crystalline base materials from other classes of compounds, for example, in order to influence the dielectric and/or optical anisotropy of such a dielectric and/or to optimise its threshold voltage and/or its viscosity.
The compounds of formula I are colorless in their pure state and form liquid-crystalline mesophases in a temperature range favorable for electro-optical use. In particular, the compounds according to the invention are distinguished by their high clearing point and their low rotational viscosity values. They are stable chemically, thermally, and with respect to light.
The invention particularly relates to compounds of formula I, where R1 is alkyl having from 1 to 10 C atoms, or an alkenyl radical having from 2 to 10 C atoms.
Preferred are compounds of formula I where c is 0. Z1 and Z2 are preferably a single bond, or alternatively xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94C2F4xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94 or xe2x80x94COOxe2x80x94. Preferably a is 0.
If R1 and/or R2 is an alkyl radical and/or an alkoxy radical, this can be straight-chain or branched. Preferably it is straight-chain, has 2, 3, 4, 5, 6 or 7 C atoms, and therefore preferably is ethyl, propyl, butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy, heptoxy, methyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, methoxy, octoxy, nonoxy, decoxy, undecoxy, dodecoxy, tridecoxy or tetradecoxy.
Oxaalkyl preferably is straight-chain 2-oxapropyl (=methoxymethyl), 2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or 4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl, or 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.
If R1 and/or R2 is an alkenyl radical, it can be straight-chain or branched. Preferably it is straight-chain and has from 2 to 10 C atoms. It is therefore, in particular, vinyl, prop-1-, or prop-2-enyl, but-1-, 2- or but-3-enyl, pent-1-, 2-, 3- or pent-4-enyl, hex-1-, 2-, 3-, 4- or hex-5-enyl, hept-1-, 2-, 3-, 4-, 5-, or hept-6-enyl, oct-1-, 2-, 3-, 4-, 5-, 6- or oct-7-enyl, non-1-, 2-, 3-, 4-, 5-, 6-, 7- or non-8-enyl, dec-1-, 2-, 3-, 4-, 5-, 6-, 7-, 8- or dec-9-enyl.
If R1 and/or R2 is an alkyl radical, in which one CH2 group has been replaced by xe2x80x94Oxe2x80x94 and one by xe2x80x94COxe2x80x94, these are preferably adjacent. These therefore comprise an acyloxy group xe2x80x94COxe2x80x94Oxe2x80x94 or an oxycarbonyl group xe2x80x94Oxe2x80x94COxe2x80x94. Preferably, they are straight-chain and have from 2 to 6 C atoms.
In particular, they therefore are acetyloxy, propionyloxy, butyryloxy, pentanoyloxy, hexanoyloxy, acetyloxymethyl, propionyloxymethyl, butyryloxymethyl, pentanoyloxymethyl, 2-acetyloxyethyl, 2-propionyloxyethyl, 2-butyryloxyethyl, 3-acetyloxypropyl, 3-propionyloxypropyl, 4-acetyloxybutyl, methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl, ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxycarbonylmethyl, 2-(methoxycarbonyl)ethyl, 2-(ethoxycarbonyl)ethyl, 2-(propoxycarbonyl)ethyl, 3-(methoxycarbonyl)propyl, 3-(ethoxycarbonyl)propyl, or 4-(methoxycarbonyl)butyl.
If R1 and/or R2 is an alkyl or alkenyl radical singly substituted by CN or CF3, said radical is preferably straight-chain. The substitution by CN or CF3 can be in any position.
If R1 and/or R2 is an alkyl or alkenyl radical at least singly substituted by halogen, said radical is preferably straight-chain and halogen is preferably F or Cl. In the case of polysubstitution, halogen is preferably F. The resulting radicals also include perfluorinated radicals. In the case of single substitution, the fluoro or chlorine substituent can be in any position, but preferably in the xcfx89-position.
Compounds of formula I carrying branched pendant groups R1 and/or R2 are occasionally of interest because of their better solubility in the conventional liquid-crystalline base materials. In particular, however, they are of interest as chiral dopants if they are optically active. Smectic compounds of this type are suitable as components for ferroelectric materials.
Compounds of formula I having SA phases are suitable, for example, for thermally addressed displays.
Branched groups of this type as a rule do not contain more than one chain branching. Preferred branched radicals R1 and R2 are isopropyl, 2-butyl (=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl (=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl, 2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy, 3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, 1-methylhexoxy, 1-methylheptoxy.
R2 is preferably H, F, Cl, CN, CF3, SF5, CF2H, OCF3, OCF2H, OCFHCF3, OCFHCFH2, OCFHCF2H, OCF2CH3, OCF2CFH2, OCF2CF2H, OCF2CF2CF2H, OCF2CF2CFH2, OCFHCF2CF3, OCFHCF2CF2H, OCFHCFHCF3, OCH2CF2CF3, OCF2CF2CF3, OCF2CFHCFH2, OCF2CH2CF2H, OCFHCF2CFH2, OCFHCFHCF2H, OCFHCH2CF3, OCH2CFHCF3, OCH2CF2CF2H, OCF2CFHCH3, OCF2CH2CFH2, OCFHCF2CH3, OCFHCFHCFH2, OCFHCH2CF3, OCH2CF2CFH2, OCH2CFHCF2H, OCF2CH2CH3, OCFHCFHCH3, OCFHCH2CFH2, OCH2CF2CH3, OCH2CFHCFH2, OCH2CH2CF2H, OCHCH2CH3, OCH2CFHCH3, OCH2CH2CF2H, OCClFCF3, OCClFCClF2, OCClFCFH2, OCFHCCl2F, OCClFCF2H, OCClFCClF2, OCF2CClH2, OCF2CCl2H, OCF2CCl2F, OCF2CClFH, OCF2CClF2, OCF2CF2CClF2, OCF2CF2CCl2F, OCClFCF2CF3, OCClFCF2CF2H, OCClFCF2CClF2, OCClFCFHCF3, OCClFCClFCF3, OCCl2CF2CF3, OCClHCF2CF3, OCClFCF2CF3, OCClFCClFCF3, OCF2CClFCFH2, OCF2CF2CCl2F, OCF2CCl2CF2H, OCF2CH2CClF2, OCClFCF2CFH2, OCFHCF2CCl2F, OCClFCFHCF2H, OCClFCClFCF2H, OCFHCFHCClF2, OCClFCH2CF3, OCFHCCl2CF3, OCCl2CFHCF3, OCH2CClFCF3, OCCl2CF2CF2H, OCH2CF2CClF2, OCF2CClFCH3, OCF2CFHCCl2H, OCF2CCl2CFH2, OCF2CH2CCl2F, OCClFCF2CH3, OCFHCF2CCl2H, OCClFCClFCFH2, OCFHCFHCCl2F, OCClFCH2CF3, OCFHCCl2CF3, OCCl2CF2CFH2, OCH2CF2CCl2F, OCCl2CFHCF2H, OCClHCClFCF2H, OCF2CClHCClH2, OCF2CH2CCl2H, OCClFCFHCH3, OCF2CClFCCl2H, OCClFCH2CFH2, OCFHCCl2CFH2, OCCl2CF2CH3, OCH2CF2CClH2, OCCl2CFHCFH2, OCH2CClFCFCl2, OCH2CH2CF2H, OCClHCClHCF2H, OCH2CCl2CF2H, OCClFCH2CH3, OCFHCH2CCl2H, OCClHCFHCClH2, OCH2CFHCCl2H, OCCl2CH2CF2H, OCH2CCl2CF2H, CHxe2x95x90CF2, CFxe2x95x90CF2, OCHxe2x95x90CF2, OCFxe2x95x90CF2, CHxe2x95x90CHF, OCHxe2x95x90CHF, CFxe2x95x90CHF, OCFxe2x95x90CHF, especially F, Cl, CN, CF3, SF5, CF2H, OCF3, OCF2H, OCFHCF3, OCFHCFH2, OCFHCF2H; OCF2CH3, OCF2CFH2, OCF2CF2H, OCF2CF2CF2H, OCF2CF2CFH2, OCFHCF2CF3, OCFHCF2CF2H, OCF2CF2CF3, OCF2CHFCF3, OCClFCF2CF3.
For the sake of simplicity, hereinafter Cyc represents a 1,4-cyclohexylene radical, Che a 1,4-cyclohexenyl radical, Dio a 1,3-dioxane-2,5-diyl radical, Dit a 1,3-dithiane-2,5-diyl radical, Phe a 1,4-phenylene radical, Pyd a pyridine-2,5-diyl radical, Pyr a pyrimidine-2,5-diyl radical, Bi a bicyclo[2.2.2]octylene radical, PheF a 2- or 3-fluoro-1,4-phenylene radical, PheFF a 2,3-difluoro- or 2,6-difluoro-1,4-phenylene radical, Nap a substituted or unsubstituted naphthalene radical, Dec a decahydronaphthalene radical.
The compounds of the formula I accordingly comprise the preferred compounds having three rings of the subformulae Ia to If:
Compounds having four rings of the subformulae Ik to Iw:
Particularly preferred among these are the compounds of the subformulae Ia, Ib and Ic.
In the compounds of the previous and following formulae, R2 is preferably F, CN, OCF3, OCHF2, CF3, OCHFCF3, OC2F5 or OCF2CHFCF3, straight-chain alkyl or alkoxy.
R1 is preferably straight-chain, unsubstituted alkyl, alkoxy, alkenyloxy or alkenyl having up to 10 C atoms.
A2 is preferably Phe, PheF, PheFF, Cyc or Che, also Pyr or Dio, Dec or Nap. Preferably the compounds of formula I do not include more than one of the radicals Bi, Pyd, Pyr, Dio, Dit, Nap or Dec.
Preferred are compounds of formula I in which A1 is a singly or doubly substituted 1,4-phenylene. In particular, these are 2-fluoro-1,4-phenylene, 3-fluoro-1,4-phenylene, 2,3-difluoro-1,4-phenylene and 2,6-difluoro-1,4-phenylene.
Preferred smaller groups of compounds of formula I are those of the subformulae I1 to I24: 
where
R1 has the meanings described previously and xe2x80x9calkylxe2x80x9d is a straight-chain or branched alkyl having 1-9 C atoms.
The compounds of the formula I are prepared in accordance with methods known per se, as described in the literature (e.g. in the standard textbooks such as Houben-Weyl, Methoden der Organischen Chemie, Georg-Thieme-Verlag, Stuttgart) specifically under reaction conditions known and suitable for these reactions. Variants known per se but not mentioned here in detail are also included.
The compounds according to the invention can, for example, be prepared as follows:
(Rxe2x80x2 is alkyl; L1 and L2 are each, independently, H or R) 
The invention also relates to electro-optical displays (in particular STN displays or matrix LCDs with two plane-parallel substrates which, together with a border, form a cell, which have integrated nonlinear elements for switching individual pixels on the substrates and, wherein a nematic liquid crystal mixture having positive dielectric anisotropy and high resistivity is present in the cell) which comprise media as described previously and to the use of these media for electro-optical purposes.
The liquid crystal mixtures according to the invention permit considerable expansion of the available parameter space.
The media according to the invention achieve combinations of clearing point, optical anisotropy, viscosity at low temperature, thermal and UV stability and dielectric anisotropy far superior to current prior art materials.
So far it has not been possible to adequately meet the requirements of a high clearing point, a nematic phase at low temperature and a high xcex94∈. While liquid-crystal mixtures such as e.g. MLC-6476 and MLC-6625 (Merck KGaA, Darmstadt, Germany) do exhibit comparable clearing points and low temperature stabilities, they have relatively high xcex94n values as well as higher threshold voltages of about xe2x89xa71.7 V.
Other mixture systems have comparable viscosities and values of xcex94∈, but have clearing points around 60xc2x0 C.
The liquid crystal mixtures according to the invention, while maintaining the nematic phase down to xe2x88x9220xc2x0 C., preferably down to xe2x88x9230xc2x0 C., and particularly preferably down to xe2x88x9240xc2x0 C., make it possible to achieve clearing points above 80xc2x0 C., preferably above 90xc2x0 C., and particularly preferably above 100xc2x0 C., with simultaneous dielectric anisotropy values of xcex94∈xe2x89xa74, and preferably xe2x89xa76 and a high specific resistivity, which makes possible excellent STN displays and matrix LCDs. The mixtures in particular are characterized by low operating voltages. The TN thresholds are below 1.5 V, preferably below 1.3 V.
It follows that with suitable choices for the components of the mixtures according to the invention one can achieve higher clearing points (e.g. above 110xc2x0 C.) in conjunction with a higher threshold voltage, or lower clearing points in conjunction with lower threshold voltages while maintaining other advantageous properties. It is equally possible, in conjunction with a correspondingly small increase in viscosities, to obtain mixtures with a larger xcex94∈ and consequently lower thresholds. The matrix LCDs according to the invention preferably operate in the first transmission minimum according to Gooch and Tarry [C. H. Gooch and H. A. Tarry, Electron. Lett. 10, 2-4, 1974; C. H. Gooch and H. A. Tarry, Appl. Phys. Vol. 8, 1575-1584, 1975], resulting not only in particularly favorable electro-optical properties such as e.g. steep slope of the characteristic curve and low angular dependence of contrast (DE-C 3022818), but also a smaller dielectric anisotropy being sufficient in the second minimum in conjunction with a threshold voltage equal to that of an analogue display. Consequently it is possible, using the mixtures according to the invention, to achieve distinctly higher resistivities in the first minimum than with mixtures comprising cyano compounds. Those skilled in the art, using simple routine methods, are able, via a suitable choice of the individual components and their proportions by weight, to adjust the birefringence required for a predefined layer thickness of the matrix LCD.
The flow viscosity xcexd20 at 20xc2x0 C. is preferably  less than 60 mm2xc2x7sxe2x88x921, particularly preferably  less than 50 mm2xc2x7sxe2x88x921. The nematic phase domain is preferably at least 90xc2x0, in particular at least 100xc2x0. Preferably, this domain extends at least from xe2x88x9230xc2x0 to +80xc2x0.
Measurements of the xe2x80x9ccapacity holding ratioxe2x80x9d (HR) [S. Matsumoto et al., Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SID Conference, San Francisco, June 1984, p. 304 (1984); G. Weber et al., Liquid Crystals 5, 1381 (1989)] have shown that mixtures according to the invention comprising compounds of the formula I exhibit a distinctly smaller decrease in HR with increasing temperature than analogous mixtures comprising, instead of the compounds of the formula I, cyanophenylcyclohexanes of the formula 
or esters of the formula 
The UV stability of the mixtures according to the invention is likewise considerably better, i.e. they exhibit a distinctly smaller decrease in HR under exposure to UV.
Preferably, the media according to the invention are based on a plurality of (preferably two, three, four or more) compounds of formula I, i.e. the proportion of these compounds is 5-95%, preferably 10-60% and particularly preferably in the range of 15-40%.
Individual compounds of the formulae I to X (formulae II to X are described below) and their subformulae which can be used in the media according to the invention are either known or can be prepared in a manner similar to that of known compounds.
Preferred embodiments are specified below:
The medium preferably comprises 1, 2 or 3 homologous compounds of formula I, no more than 10% of each homologue being present in the mixture.
The medium comprises compounds of formula I where R1 is preferably ethyl and/or propyl, alternatively butyl, pentyl, hexyl and heptyl. Compounds of formula I having short side chains R1 have a positive effect on the elastic constants, especially K1, and result in mixtures having particularly low threshold voltages.
The medium additionally comprises one or more compounds of formulae II to X: 
xe2x80x83where the individual radicals have the following meanings:
R0 is n-alkyl; oxaalkyl, fluoroalkyl, alkenyloxy or alkenyl, each having up to 9 C atoms,
X0 is halogenated alkyl, halogenated alkenyl, halogenated alkenyloxy, halogenated alkoxy, each having up to 7 C atoms, F or Cl,
Z0 is xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94C2H4xe2x80x94, xe2x80x94C2F4xe2x80x94, xe2x80x94CFxe2x95x90CFxe2x80x94, xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94 or xe2x80x94COOxe2x80x94,
Y1, Y2, Y3 and Y4 are, each independently, H or F, and
r is 0 or 1.
The compound of formula IV is preferably 
The medium additionally comprises one or more compounds of the formulae 
wherein R0 and Y2 are as previously defined.
The medium preferably comprises one, two or three, alternatively four homologues of the compounds of H1 to H17 (n=1-7): 
The medium additionally comprises one or more dioxanes of the formula DI and/or DII, 
wherein R0 is n-alkyl, oxaalkyl, fluoroalkyl, alkenyloxy or alkenyl, each having up to 9 C atoms. Preferably R0 in the compounds of formula DI and/or DII is straight-chain alkyl or alkenyl having up to 7 C atoms.
The medium additionally comprises one or more compounds of formulae XI to XVI: 
wherein R0 is n-alkyl, oxaalkyl, fluoroalkyl, alkenyloxy or alkenyl, each having up to 9 C atoms. X0 is F, Cl, halogenated alkyl, hologenated alkenyl, halogenated alkenyloxy or halogenated alkoxy having up to 7 C atoms. Y1, Y2, Y3 and Y4 are, each independently, H or F. X0 preferably is F, Cl, CF3, OCF3, OCHF2. R0 preferably is alkyl, oxaalkyl, fluoroalkyl, alkenyl or alkenyloxy.
The proportion of compounds of formulae I to X together in the overall mixture is at least 50 wt %.
The proportion of compounds of formula I in the overall mixture is from 5 to 50 wt %.
The proportion of compounds of formulae II to X in the overall mixture is from 30 to 70 wt %. 
The medium comprises compounds of formulae II, III, IV, V, VI, VII, VIII, IX and/or X.
R0 is straight-chain alkyl or alkenyl of 2 to 7 C atoms.
The medium essentially comprises compounds of the formulae II to XVI.
The medium comprises further compounds, preferably of formulae XVII to XX: 
wherein R0 and X0 are as previously defined and the 1,4-phenylene rings can be CN-, chloro- or fluorosubstituted. Preferably, the 1,4-phenylene rings are singly substituted or polysubstituted by fluorine atoms.
The medium comprises further compounds, preferably of formulae RI to RX, 
xe2x80x83wherein
R0 is n-alkyl, oxaalkyl, fluoroalkyl, alkenyloxy or alkenyl each having up to 9 C atoms,
d is 0, 1 or 2,
Y1 is H or F,
Alkyl or
Alkyl* are, each independently, a straight-chain or branched alkyl radical having 1-9 C atoms,
Alkenyl or
Alkenyl* are, each independently, a straight-chain or branched alkenyl radical having up to 9 C atoms.
The medium preferably comprises one or more compounds of the formulae 
wherein n and m are each an integer of 1 to 9.
The proportion by weight of compounds of formula I: compounds of formulas (II+IIII+IV+V+VI+VII+VIII+IX+X) together is preferably from 1:10 to 10:1.
The medium essentially comprises compounds of formulae I to XVI.
It was found that even a relatively small proportion of the compounds of formula I mixed with conventional liquid crystal materials, but in particular with one or more compounds of formula II, III, IV, V, VI, VII, VIII, IX and/or X leads to a considerable decrease in the threshold voltage and to low values of the birefringence, wide-domain nematic phases with low smecticnematic transition: temperatures, thereby improving the storage stability. The compounds of the formulae I to X are colorless, stable and readily miscible with one another and with other liquid crystal materials.
The term xe2x80x9calkylxe2x80x9d or xe2x80x9calkylxe2x80x9d encompasses straight-chain and branched alkyl groups having 1-9 carbon atoms, particularly the straight-chain groups methyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Groups having 2-5 carbon atoms are preferred.
The term xe2x80x9calkenylxe2x80x9d or xe2x80x9calkenylxe2x80x9d encompasses straight-chain and branched alkenyl groups having up to 9 carbon atoms, preferably the straight-chain groups. Preferred alkenyl groups are C2-C7-1E-alkenyl, C4-C7-3E-alkenyl, C5-C7-4-alkenyl, C6-C7-5-alkenyl and C7-6-alkenyl, especially preferred are C2-C7-1E-alkenyl, C4-C7-3E-alkenyl and C5-C7-4-alkenyl. Examples of preferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5 carbon atoms are preferred.
The term xe2x80x9cfluoroalkylxe2x80x9d preferably comprises straight-chain groups with terminal fluorine, i.e., fluoromethyl, 2-fluoroethyl, 3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl. Other positions of fluorine are not precluded, however.
The term xe2x80x9coxaalkylxe2x80x9d preferably comprises straight-chain radicals of the formula CnH2n+1xe2x80x94Oxe2x80x94(CH2)m, where n and m each, independently of one another, are 1 to 6. Preferably, n is 1 and m is 1 to 6.
By varying the choice for R0 and X0, one can modify the response times, the threshold voltage, the slope of the transmission characteristics etc. as desired. For example, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxy radicals and the like as a rule lead to shorter response times, improved nematic tendencies and a higher ratio of the elastic constants k33 (bend) and k11 (splay), compared with alkyl or alkoxy radicals. 4-Alkenyl radicals, 3-alkenyl radicals and the like generally result in lower threshold voltages and smaller values of k33/k11 than alkyl and alkoxy radicals.
A xe2x80x94CH2CH2xe2x80x94 group in Z1 generally leads to higher values of k33/k11, compared to a single covalent bond. Higher values of k33/k11 permit, for example, less steep transmission characteristics in TN cells with twists of 90xc2x0 (to achieve grey hues) and steeper transmission characteristics in STN, SBE and OMI cells (higher multiplexability) and vice versa.
The optimal quantitative ratio of the compounds of the formula I to formulae II+III+IV+V+VI+VII+VIII+IX+X largely depends on the desired characteristics, on the choice of components for the formulae I, II, III, IV, V, VI, VII, VIII, IX and/or X and on the choice of any further components. Suitable quantitative ratios within the above-specified range can readily be determined ad hoc.
The total quantity of compounds of the formulae I to XVI in the mixtures according to the invention is not critical. The mixtures can therefore comprise one or more further components, to optimize various properties. The observed effect on the response times and on the threshold voltage, however, is as a rule higher, the higher the overall concentration of compounds of the formulae I to XVI.
In a preferred embodiment, the media according to the invention comprise compounds of the formulae II to X (preferably II and/or III), where X0 is OCF3, OCHF2, F, OCHxe2x95x90CF2, OCFxe2x95x90CF2, OCF2CHFCF3 or OCF2xe2x80x94CF2H. A beneficial synergistic effect with the compounds of formula I results in particularly advantageous properties.
The mixtures according to the invention having low optical anisotropy (xcex94n less than 0.07) are particularly suitable for reflective displays. Low Vth mixtures are especially suitable for 3.3 V drivers and also for 4 V or 5 V drivers. Mixtures free from esters are preferred for the latter applications.
In the present application and in the following examples, the construction of the matrix LCD according to the invention comprising polarizers, electrode baseplates and electrodes with a surface treatment corresponds to the standard design of such displays. Within the present context, the term xe2x80x9cstandard designxe2x80x9d is comprehensive and additionally covers any variations and modifications of the matrix LCD, including in particular matrix display elements on the basis of poly-Si TFT or MIM.
An essential difference between the displays according to the invention and the current customary displays based on the twisted nematic cell is the choice of liquid crystal parameters of the liquid crystal layer.
The preparation of the liquid crystal mixtures which can be used according to the invention is carried out by methods which are customary per se. As a rule, the desired quantity of the components used in smaller amounts is dissolved in the component which constitutes the main ingredient, preferably at an elevated temperature. An alternative procedure is to mix solutions of the components in an organic solvent, e.g. in acetone, chloroform or methanol, and then, after thorough mixing, to remove the solvent again, for example by distillation.
The dielectrics can also comprise further additives known to those skilled in the art and described in the literature. For example, 0-15% of pleochroic dyes or chiral dopants can be added.
C refers to a crystalline phase, S to a smectic phase, Sc to a smectic phase, N to a nematic phase and I to the isotropic phase.
V10 denotes the voltage for 10% transmission (viewing direction perpendicular to the substrate surface). ton denotes the on and toff the off time at an operating voltage corresponding to 2.0 times the value of V10. xcex94n denotes the optical anisotropy and n0 the refractive index. xcex94∈ denotes the dielectric anisotropy (xcex94∈=∈∥-∈xe2x8axa5, where ∈∥ refers to the dielectric constant parallel to the longitudinal axes of the molecule and ∈xe2x8axa5 is the dielectric constant perpendicular thereto). The electro-optical data were measured in a TN cell in the 1st minimum (i.e. at a dxc2x7xcex94n value of 0.5) at 20xc2x0 C., unless explicitly stated otherwise. The optical data were measured at 20xc2x0 C., unless explicitly stated otherwise.
In the present application and in the following examples the structures of the liquid crystal compounds are specified by acronyms, which can be transformed into chemical formulae according to the following Tables A and B. All the radicals CnH2n+1 and CmH2m+1 are straight-chain alkyl radicals having n or m C atoms n and m, independently of one another, denote 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or 15. The coding according to Table B is self-evident. Table A specifies the acronym for the parent body only. In individual cases, the acronym for the parent body is followed, separated therefrom by a hyphen, by a code for the substituents R1, R2, L1 and L2:
Preferred mixture components are listed in Tables A and B.
Table C lists possible dopants which can be added to the mixtures according to the invention.
Table D lists exemplary stabilizers which can be added to the mixtures according to the invention.
The following examples are intended to illustrate the invention without limiting it. Hereinbefore and hereinafter, percentages are given in per cent by weight. All temperatures are specified in degrees centigrade. M.p. means melting point, c.p. means clearing point, C means crystalline state, N means nematic phase, S means smectic phase, and I means isotropic phase. Data appearing between these symbols represent the transition temperatures. xcex94n means optical anisotropy (589 nm, 20xc2x0 C.). The flow viscosity xcexd20 (mm2/sec) and the rotational viscosity xcex31 [mPaxc2x7s] were each determined at 20xc2x0 C.
xe2x80x9cStandard work-upxe2x80x9d means: water is added if required, the mixture is extracted with dichloromethane, diethyl ether, methyl tert-butyl ether or toluene, followed by phase separation, then drying of the organic phase, evaporation of the solvent and purification of the product by distillation under reduced pressure or crystallization and/or chromatography. The following abbreviations are used:
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
The entire disclosure of all applications, patents and publications, cited above or below, and of corresponding German application No. 10064996.3, filed Dec. 23, 2001 is hereby incorporated by reference.